Crossover Pipe

[mpe]

I was wondering if anyone had an opinion on how removing the crossover pipe and plugging the holes on the exhaust would affect engine performance. Adjusting the carburetors would be much easier without the crossover tube. Thanks.

[Burt]

Have you not used the search function? 

There is one topic which may have some of the information you seek.  X-overectomy 

[Barry]

It's been done at least once on this forum. The late Monte Millar did it for certain which might help yours search.

The theory goes that it will impact low rev torque but whenever I've heard reports of it being done they say they can't detect any difference. You would need to see a before and after on a dyno to know for sure.

It's an expensive thing for manufacturers to do so you would think they do it for a good reason.  Besides modifying the torque curve crossovers also have benefits in exhaust noise reduction with some manufacturers saying they couldn't get below the maximum permitted noise levels without a crossover.

[mpe]

Thank you for the information. I did some research and found this crossover is called an H pipe. Apparently it equalizes exhaust pulses and improves low end torque but does not improve scavenging. I don't understand the science and engineering behind all of that but in theory the pipe has a reason for being there. These Bing flat top carburetors seem to be a bit finicky and I thought they might be a bit easier to tune them without the pipe. I've always tuned twin carbs by the exhaust note but the pipe is balancing the exhaust so I can't hear each cylinder individually. At least it four carburetors.

Motortrend says;
"On a typical 400-horsepower street car, adding an X-pipe or an H-pipe to an existing dual exhaust system can add 7-12 hp. A smoother exhaust note and reduced interior resonance are added bonuses."

Would any difference be noticeable on a two cylinder, 45 HP engine? Probably not but who knows.

[Barry]

Yes flat tops are something of an enigma in that you wonder why they went with that experiment when they only lasted the two years from 78 - 80.  There's a tendency to assume they were inadequate in some way and that was the reason for reverting back to dome tops from 81.  Mine has flat tops but I can't say they have been any trouble at all over the last 17 years. I balance them with a manometer and occasionally check the idle balance using the shorting method.

[mpe]

I cleaned and put rebuild kits in the carburetors. They were in pretty good shape for sitting ten years. The floats are sticking sometimes so they need to come off again to get to the cause of that. Or maybe they will stop on their own. Other than that they seem to work well enough for 44 year old carburetors.

[mpe]

The carbs are 64/32/204 right side, 64/32/203 left side. They apparently came from the factory without the vacuum test ports being drilled so they can't be synced with a manometer or a carb stick. After some tinkering and precision guesswork they seem to be working well enough. 

[Barry]

That's odd, my carbs are slightly older than yours being 201's and 202's and yet they do have the vacuum ports.

For setting the balance at idle the shorting method works well. 

For throttle cable synch, as well as vacuum balance, I devised a very sensitive mechanical method.

I pierced two short lengths of blue rubber tube with a pair of fine knitting needles (because they are stiff and very lightweight alloy) and pushed the rubber tube on to the inside of the throttle spindles.  The needles amplifies the rotary movement of the throttle spindle. I can sit astride the bike and open the throttle very slowly to see if one needle twitches before the other.  The engine runs very smoothly with this method alone and it compares very closely with vacuum balancing.

 

[mpe]

Thank you, I will try that. I have some bamboo skewers that might work. Very odd that these carbs were not drilled for vacuum ports. Maybe the BMW shops drilled them when a person brought their bike in for it's first scheduled maintenance. I am considering drilling them and adding a pipe and a plug, shouldn't be too difficult but the slides would have to come out to remove the debris, wouldn't want the aluminium bits getting into the engine.

I've tinkered with the carbs and they seem to be balanced reasonably well. It idles at 1000 RPM, powers on smoothly, no backfire on deceleration. I've owned many motorcycles but is the first BMW so I'm still gathering information on them. It's always a learning curve with a new brand.

Later that day:
I went out this afternoon to explore the 'knitting needle' method of balancing the carbs. Could not find the appropriate size or flexibility of hose, didn't have knitting needles plus I couldn't get my fat fingers in that tight space to attach the hose anyway. Came up with a alternate plan using bamboo skewers. Shoved the pointed end into the gap where the barrel is on the end of the cable at an appropriate angle. Twisted the throttle slightly, left skewer moved, right one didn't. They weren't too far off, did a bit of adjusting on the barrels from side to side until they were both just right. Fired up the engine, noticed a slight difference...every little bit helps. Bamboo skewers...what can't they do?

[Barry]

Here's something to bear in mind. If you have done some research on airheads you might have come across a guy call Duane Ausherman. he had a BMW dealership and was a wise sage on airheads. Below is what he had to say about carb tuning. It took me a long time but after many years I've come around to this way of thinking and now very rarely fuss of carb balance.

http://w6rec.com/why-it-is-hard-to-tune-a-bmw-twin/


The bottom line on carb tuning for the BMW twin
A good BMW twin tuner with a lot of experience will rarely find a perfectly tuned engine.  Each of my mechanics went through a “crazy period” when they were learning to tune carbs.  While riding their bikes, they would reach down every few minutes and do a slight tweak on a carb.  Eventually, they learned to accept that it would never be perfect.  The very best that you will ever achieve is a workable compromise.  Be happy.

[mpe]

And here I though I had purchased an example of quality, precision German engineering. It's very disappointing to discover that, according to some, it is a virtually un-tunable piece of crap.

[Bob_Roller]

Commonly referred to as a ‘farm implement ‘. ‘

[dogshome]

I built an electronic balancer. I use it to get the best idle on the screws, and same at pickup as the cables start to open the butterflies.

This gives me the best response around town and coming out of slow bends. I can't get it as good any other way. Silent hectik ignition also had an appreciable effect on getting a good idle and pickup. My bean can worked fine, but had done 60,000 miles and there was some small shimmy using a timing light. No worse than an old Leyland mini and a rebuild showed it's still in good condition. Electronic off the crankshaft is obviously more consistent.

Are they balanced at every RPM further up? Probably not, but it isn't noticeable to me.